One complaint voiced by many television viewers is the changes in volume viewers endure during commercial breaks and when switching between different channels. Similar volume extremes may also occur with other devices, such as portable audio players, A/V receivers, personal computers, and vehicle audio systems. One solution for this problem is automatic gain control (AGC). A typical automatic gain control (AGC) works by reacting to volume changes by cutting an audio signal at high amplitude and then boosting it at low amplitude—no matter where in the frequency range the loudness spike occurs.
When the AGC kicks in, unwanted changes and unnatural artifacts can often be heard in the form of pumping and breathing fluctuations. Pumping fluctuations can be the result of bass tones disappearing when the loudness suddenly increases, like during a loud action sequence. Breathing fluctuations can happen when low level hiss is boosted during quiet passages. Unfortunately, this brute force method of handling volume changes does not take into account how humans actually perceive change in volume.